Dave,
I think if you make sure you have the correct tension on the belt, you should know what i mean you have run belt cars before, in this sense this one is no different.
I think the belt (drive gear) roller is a must, it just gives you an extra safe guard, while it is ok for the belt to skip a tooth here and there, it is nicer for them not to skip at all
. Maybe its just me but in you initial post you seemed to imply that this roller will add tension to the belt. It will not, it just sits as close to the drive gear as you can get it without it actually touching so if the belt does lift and try to skip, the roller will not let it.
If the belt is skipping it will sound like a clicking sound if its only 1 or 2 teeth at a time, if its more then it will sound like it is ripping. Thats probably the teeth on your belt
(joke)
You say it sounds like it is slipping, personally i would tighten the slipper right up so you know its not the slipper, then set the diff (fully tighten then back off 1/8th of a turn for starters. and try it.
Its just a process of elimination.
You should have some backlash in the teeth on between the pinion and spur like any other car